1. Field of the Invention
This invention relates to the materials and a method for fabricating multilayer nonlinear dielectric optical structures from polymeric materials. In particular, the present invention relates to multilayer materials characterized by a modulation in the nonlinear refractive index in the direction normal to the surface of the layers.
2. Discussion of the Related Art
The propagation of electromagnetic waves through periodic structures consisting of layers of materials with an intensity-dependent dielectric constant has been studied both theoretically and experimentally. A historical review was reported by Brown et al (T. G. Brown and B. J. Eggleton, Optics Express 3, 385 (1998)). The transmission and reflection properties of such structures are strongly modulated by the intensity of the incident light. The optical response of such structures can include optical switching, optical limiting, optical bistability and some remarkable pulse propagation effects including transverse pattern formation.
Some optical effects have been shown experimentally using, for example, colloidal arrays (C. Herbert and M. Malcuit.; Opt. Lett. 17, 1037 (1992)), semiconductor multilayers, silicon-on-insulator waveguides (N. D. Sankey, D. F. Prelewitz and T. G. Brown; Appl. Phys. Lett. 60, 1427 (1992)) and fiber gratings (S. Larochelle, Y. Hibino, V. Mizrahi and G. Stegeman; Electron. Lett. 26, 1459 (1990)). An organic nonlinear dielectric stack was reported by Norwood et al. in 1992 using a silicon naphthalocyaine-poly(methyl methacrylate) structure made by spin coating sequential layers (R. A. Norwood et al.; Opt. Lett. 17, 577 (1992)). However, only a few layers are possible with a spin coating technique. Norwood et al in the above publication reports only 23 layers.
The preparation of layered structures of polymeric materials by coextrusion has been used to prepare materials with combinations of physical properties. Such materials having three to seven layers are commercially available. More recently, the development of layer multiplying dies has allowed the preparation of multilayer polymeric materials with hundreds or even thousands of layers (E. Baer, J. Kerns, and A. Hiltner; xe2x80x9cProcessing and Properties of Polymer Microlayered Systemsxe2x80x9d: NATO-ASI on Structure Development During Polymer Processing, Guimaraes, Portugal, May 17-28, (1999)). The total thickness of the structured material is controlled by the feed ratio. With thousands of layers, individual layer thickness down to 30 nanometers or less can be achieved. Recent processing improvements and improvements in the multiplying elements allow layer thickness to be constant within a few percent.
Optical properties of nonlinear dyes have led to the development of materials that have a large nonlinear absorption and/or a large nonlinear refraction coefficients (J. S. Shirk, R. G. S. Pong, F. J. Bartoli, A. W. Snow; Appl. Phys. Lett., 63, 1880 (1993)). In these materials, the nonlinear response includes contributions from excited state absorption, excited state nonlinear refraction and thermal refraction. Some of these materials are soluble in polymeric materials that are suitable for the microlayer extrusion process.
Accordingly, there is a need to develop multilayer structures that exhibit improved nonlinear optical response.
The present invention provides a method and materials for fabrication of a multilayer structure comprising a plurality of at least two alternating layers (A) and (B) represented by formula (AB)x, where x=2n, and n is in the range of from 2 to 15. Layer (A) is comprised of component (a) and layer (B) is comprised of component (b), where at least one of components (a) and (b) exhibits nonlinear optical response.